U.S. patent number 4,572,616 [Application Number 06/406,871] was granted by the patent office on 1986-02-25 for adaptive liquid crystal lens.
This patent grant is currently assigned to Syracuse University. Invention is credited to Dennis S. Cleverly, Philipp G. Kornreich, Stephen T. Kowel.
United States Patent |
4,572,616 |
Kowel , et al. |
February 25, 1986 |
Adaptive liquid crystal lens
Abstract
A liquid crystal adaptive lens system wherein the index of
refraction profile of the liquid crystal is controlled electrically
to bring entering light to focus.
Inventors: |
Kowel; Stephen T. (Syracuse,
NY), Kornreich; Philipp G. (No. Syracuse, NY), Cleverly;
Dennis S. (Utica, NY) |
Assignee: |
Syracuse University (Syracuse,
NY)
|
Family
ID: |
23609734 |
Appl.
No.: |
06/406,871 |
Filed: |
August 10, 1982 |
Current U.S.
Class: |
349/200; 349/33;
359/255 |
Current CPC
Class: |
G02B
26/06 (20130101); G02F 1/13471 (20130101); G02F
1/29 (20130101); G02F 2203/28 (20130101); G02F
1/134309 (20130101); G02F 1/294 (20210101) |
Current International
Class: |
G02F
1/13 (20060101); G02F 1/1347 (20060101); G02F
1/29 (20060101); G02B 26/00 (20060101); G02B
26/06 (20060101); G02F 1/1343 (20060101); G02F
001/133 () |
Field of
Search: |
;350/347V,335,336,380,162.16,413,379,393 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Other References
Stone et al., "Focusing Effects in Interferometric Analysis of
Graded Index Optical Fibers", Applied Optics, vol. 14, No. 1, Jan.
1975, pp. 151-155..
|
Primary Examiner: Corbin; John K.
Assistant Examiner: Lewis; David
Attorney, Agent or Firm: Bruns and Wall
Claims
We claim:
1. An adaptive lens for focusing arbitrarily polarized light
entering the system at an image plane that includes
a pair of liquid crystal cells which are placed in series along an
axis defining the axis of the lens,
each of said cells having a pair of flat optically clear plates
that are perpendicularly aligned with the lens axis, a liquid
crystal housed between the plates and a series of radially disposed
transparent control electrodes mounted upon one of the plates that
are symmetrically positioned about the lens axis,
a first of said cells having means associated therewith for
preferentially aligning the liquid crystal housed therein in an
X-direction,
a second of said cells having means associated therewith for
preferentially aligning the liquid crystal housed therein in a
Y-direction,
said two liquid crystal cells being arranged to influence light
passing therethrough only with nematic material contained therein,
and
control means for selectively placing a predetermined voltage
values on each of said control electrodes for contouring the index
of refraction profile of the liquid crystal about the lens axis to
obtain a spherical lens response for focusing arbitrarily polarized
light at an image plane.
2. The lens of claim 1 wherein the two cells are mounted in
abutting contact.
3. The lens of claim 1 wherein the control means further includes
an adjusting means for selectively varying the voltage values on
the individual electrodes.
4. The lens of claim 1 wherein said control means includes a
processor for placing a predetermined voltage upon each of the
control electrodes in response to a given program.
5. An adaptive lens having a central axis for controlling the phase
of light passing through a dominant aperture of the lens that
includes
a first unit for influencing incoming light polarized in the
X-direction that is centered upon the axis of the lens,
a second unit for influencing incoming light polarized in the
Y-direction that is centered upon the axis of the lens,
each of said units containing a pair of liquid crystal cells, each
cell further including a pair of flat optically clear plates, a
nematic liquid crystal housed between the plates and a series of
parallel aligned linear electrodes disposed within the cell,
the first unit having means associated with each cell for
preferentially aligning the liquid crystal molecules in the
X-direction and the electrodes in one of said cells being disposed
in the X-direction and the electrodes in the other cell being
disposed in the Y-direction,
the second unit having means associated with each of the cells for
preferentially aligning the liquid crystal molecules in the
Y-direction and the control electrodes contained in one of the
cells being disposed in the X-direction add the control electrodes
contained in the other of said cells being disposed in the
Y-direction, and
control means connected to each of the electrodes for selectively
placing a predetermined voltage thereon for contouring the index of
refraction profile of the liquid crystal about the axis of the lens
to produce a spherical lens response for focusing arbitrarily
polarized incoming light at a desired image plane.
6. The adaptive lens of claim 5 that further includes electrical
means for adjusting the voltage value applied to individual
electrodes to correct the lens.
7. The lens of claim 5 wherein the cells are mounted in abutting
contact along the axis of the lens.
8. The lens of claim 5 wherein said control means further includes
a processor for varying the voltage applied to each electrode in
response to a given program.
Description
BACKGROUND OF THE INVENTION
This invention relates to controlling the phase of light exiting a
liquid crystal device and, in particular, to an adaptive liquid
crystal lens that is controlled electrically.
Heretofore, most liquid crystal devices have been used for display
purposes, as for example, in digital time pieces and the like. The
device usually includes an electro-optical cell similar to that
described in U.S. Pat. No. 3,977,767. The cell typically consists
of a pair of spaced apart glass plates or windows, a series of
transparent control electrodes in a seven electrode configuration
that are disposed on the inside of the plates and a suitable liquid
crystal material sandwiched between the electrodes that responds to
a voltage applied to the electrodes. Usually, the cell is designed
so that no image information is displayed when a voltage below the
threshold voltage of the liquid crystal is applied to the
electrodes. A voltage above the saturation voltage of the material
is applied to selected electrodes which in turn changes the index
of refraction of the material in the electrodes region to create a
desired image pattern. The term threshold voltage, as herein used,
refers to some initial electrode voltage at which the liquid
crystal molecules start to react to an applied force field and
begin to reorientate themselves in the field. Saturation, on the
other hand, refers to a higher electrode voltage at which any
molecule reorientation is substantially completed and any further
increase in voltage produces little or no effect in the
material.
It should be noted that the control electrodes in most display
devices are arranged to operate at two levels. The first level is
somewhere below threshold while the second is maintained between
three and five times the threshold voltage. As a consequence, each
electrode acts as an on-off switch to produce either an image or no
image in the electrode region.
Lotspeich in U.S. Pat. No. 3,424,513 discloses an electro-optical
lens wherein incoming light is caused to pass through a Kerr effect
substance that is under the influence of at least one quadrapole
control unit. The elongated electrodes of the unit are placed
parallel to the optical axis of the lens and are electrically
interconnected so that a variable control field is established with
the Kerr effect material. Because of the quadrapole arrangement, an
entering light beam can only be diverged by the lens with the
amount of divergence being dependent upon the voltage applied to
the electrodes. Normally, the control voltage must be about 20,000
volts in order to produce the desired effects. Because of the high
voltages involved and the fact that the device by itself cannot act
as a converging lens, the lens is of little or no practical
value.
Bricot et al in U.S. Pat. No. 4,037,929 describes the use of a
nematic liquid crystal to modify a glass lens. The glass lens
consists of a plano element and a convex element between which the
liquid crystal is stored. A first transparent solid area electrode
is mounted on the inside surface of the flat element and a second
similar type electrode is mounted on the inside surface of the
convex element. As taught by Bricot et al, the voltage applied to
the electrodes is varied between the threshold and saturation
voltages of the liquid crystal to change the index of refraction
thereof. By changing the index of refraction of the material the
focal length of the glass lens can be modified within extremely
narrow limits. The lens, however, suffers from all the defects
common to glass lenses and because of the two electrode
configurations, the lens is restricted to use in conjunction with
incoming light that is polarized in one direction only.
SUMMARY OF THE INVENTION
It is an object of the present invention to improve adaptive lenses
in general.
A further object of the present invention is to produce an
electronically controlled lens from a simple liquid crystal cell by
selectively varying the index of refraction of the liquid crystal
across the cell whereby the lens response approaches that of thin
lens.
Another object of the present invention is to provide a liquid
crystal lens whose index of refraction is electrically controlled
point to point on the lens to provide for near diffraction limited
performance.
A still further object of the present invention is to provide a
liquid crystal lens that can be electrically corrected for both
internal and external aberrations.
Yet another object of the present invention is to provide a liquid
crystal adaptive lens that can focus arbitrarily polarized light
electronically using a very low voltage drive and thus eliminating
the need for mechanical linkages and the like.
These and other objects of the present invention are attained by
means of a lens system having at least one electrooptical cell
containing a pair of spaced apart flat plates, a plurality of
spaced apart transparent electrodes disposed inside the plates, a
liquid crystal material contained between the plates in the
electroded region, and control means for varying the voltage
applied to each of the electrodes to provide point to point control
over the index of refraction of the material to bring incoming
light to focus at a plane. In one form of the invention a plurality
of cells are staged in series so that light polarized in any
direction perpendicular to the axis of the cells is given the
necessary phase transformation to provide a sharp clean image.
BRIEF DESCRIPTION OF THE DRAWINGS
For a better understanding of these and other objects of the
present invention, reference is had to the following detailed
description of the invention which is to be read in conjunction
with the accompanying drawing, wherein:
FIG. 1 is an enlarged side view in partial section of a cell
embodying the teachings of the present invention;
FIG. 2 is a response curve illustrating the change in the index of
refraction of a specific liquid crystal used in the cell of FIG. 1
as a function of applied voltage;
FIGS. 3a-3f are schematic diagrams illustrating the orientation of
positive liquid crystal molecules as the index of refraction of the
material varies between the extraordinary index of refraction and
the ordinary index of refraction;
FIG. 4 is a block diagram showing an address system for regulating
the voltage applied to the control electrodes contained in the cell
of FIG. 1;
FIG. 5 is a circuit diagram showing circuitry for controlling the
voltage applied to each of the control electrodes used in the cell
of FIG. 1;
FIG. 6 is an exploded view in perspective showing an electrical
package for containing a cell similar to that illustrated in FIG.
1;
FIG. 7 is an enlarged perspective view showing a complete lens
system having a pair of cells containing radially disposed
symmetrical electrodes wherein the cells are staged so that
arbitrarily polarized light entering the lens system is adapted
electrically; and
FIG. 8 is also an enlarged perspective view showing a lens system
having four cells containing linear electrodes wherein the cells
are staged so that randomly polarized light entering the lens
system is adapted electrically.
DESCRIPTION OF THE INVENTION
A simple form of the invention is illustrated in FIG. 1, wherein
the lens consists of a single cell 10 which is filled with a liquid
crystal. As will be explained in greater detail below, the index of
refraction of the liquid crystal is electrically controlled to
bring a light beam entering the cell to focus and also to permit
correction of the lens for both internal and external aberrations.
The cell includes a square shaped front window 11 and a similarly
shaped rear window 12 that are mounted in spaced apart parallel
relationship by means of a frame 13. The frame is formed of Teflon
and is sealed against the inside surfaces of the windows to
establish a leakproof chamber 14 therebetween in which a liquid
crystal 18 is stored. The windows are made from optically clear
glass so that light entering the cell is able to pass through the
liquid crystal medium.
A series of linear transparent control electrodes 15--15 are
disposed across the width of the front window in spaced apart
parallel relationship. The space provided between electrodes is
about equal to the width of the electrodes. Although not shown, the
control electrodes are brought out of the cell and each electrode
independently connected to an adjustable voltage supply whereby the
voltage applied to each electrode may be selectively varied to
establish a contoured force field within the cell. A transparent
common electrode 16 covers the entire inside surface of the back
window which cooperates with each of the control electrodes to
complete the control circuitry. The electrodes are formed of
indium-tin oxide or any other suitable material using well known
filming and masking techniques. The surfaces of the electrodes that
are exposed to the liquid crystal material are further coated with
a thin film 17 of silicon dioxide which protects the electrodes
from the liquid crystal without adversely affecting the operation
of the electrodes.
The operation of the lens will be explained in reference to a
positive nematic liquid crystal. It should be clear, however, to
one skilled in the art that a negative liquid crystal may also be
similarly employed without departing from the teachings of the
invention. Nematic liquid crystals are uniaxial in that both the
axis of the molecule and the optical axis are coincident. The
material is said to be positive when the dielectric tensor
component lying along the axis of the molecule is greater than the
component positioned perpendicular to the axis. The molecules of a
positive liquid crystal therefore tend to align themselves parallel
to the direction of an applied force field while the opposite is
true of a negative liquid crystal.
To construct a cell 10 that utilizes a positive liquid crystal and
is capable of focusing incoming X-polarized light, the molecules of
the material are first placed in a preferential alignment that is
parallel to the window surfaces in the X-direction. Preferential
alignment of the molecules is achieved by using well known rubbing
techniques which causes the molecules to orientate themselves in
the X-direction when no voltage is applied to the electrode.
FIGS. 3a-3f schematically illustrate the response of the molecules
to variations in an applied field. When the voltage applied to an
associated control electrode is below the reaction threshold
voltage for the crystal material, the molecules remain in a
homogenous state as illustrated at FIGS. 3a and 3b. At this time
the molecules are in preferred alignment and the indicatrix 20 is
at the extraordinary index of refraction (n.sub.e) location. At
some voltage between threshold and saturation, the molecules
reorientate themselves at some intermediate position as shown at 3c
and 3d and the indicatrix is now at a new position in reference to
the cell axis. Finally, when the saturation voltage is reached, all
the molecules theoretically are aligned with the field and the
indicatrix is at the ordinary index of refraction (n.sub.o)
position. This condition is illustrated at FIGS. 3e and 3f. As
previously noted, applying an increased voltage to the electrodes
after saturation is reached has little or no affect on the liquid
crystal. In practice, however, total alignment of the molecules is
not attainable because of the conditions that exist at the
boundaries of the windows. In practice, therefore, the value of the
average index of refraction at saturation is always greater than
the theoretical value.
Variations in the index of refraction of a given material for a
given range of voltages can be found either experimentally or
analytically. FIG. 2 illustrates the response curve for a cell of
the type herein described that contains a well known liquid crystal
MBBA (4-methoxybenzylidine -4'-n-buthylanaline) for which n.sub.e
=1.8062 and n.sub.o =1.5616 at a wavelength .lambda.=0.5145
microns. It should be noted that saturation is reached at a voltage
about four times threshold voltage. However, the average index of
refraction value at saturation is slightly above the ordinary index
of refraction for the material represented by the dashed line
19.
To provide the instant cell illustrated in FIG. 1 with thin lens
performance characteristics, approaching plane waves 26--26 of
X-polarized light entering the lens must be given the correct phase
transformation to produce a cylindrical output wavefront 27--27
which is brought to focus at the image plane 28. Entering light
polarized in the Y-direction undergoes a constant phase delay and
is thus not influenced by the lens. Correct phase transformation is
achieved by appropriately setting the voltages applied to each of
the control electrodes so that the index of refraction across the
cell varies smoothly as graphically depicted by the dotted line
curve 25. Assuming that light passing through the cell suffers only
a phase transformation and that the light rays are paraxial, thin
lens approximations can be made for generating the desired smooth
index of refraction profile.
To find the index of refraction at some point along the index
profile to bring an incoming plane wave of light 26--26 to focus at
a desired image plane 28, it can be assumed:
where:
A is the index of refraction of the modified wave along the Z
axis;
B is a constant; and
r is the distance to any point measured from the the Z axis in an
x-y plane
For a positive liquid crystal an intermediate index of refraction
(n) under the control of the applied field has the limits
Thus the phase delay component lying along the Z axis (r=0) may be
expressed as:
Substituting in (1) with r=r.sub.m :
A+Br.sub.m.sup.2 =n.sub.i (4)
where:
n.sub.i is the maximum index of refraction at the outer periphery
of the lens and
r.sub.m is the maximum distance to the periphery from the Z
axis.
The B component can now be expressed in terms of the index of
refraction as follows: ##EQU1## The corresponding phase transfer
function is:
Substituting for n using equations (3) and (5): ##EQU2## where:
r.sub.m is the distance from the Z-axis to the outside of the
lens;
k is 2.pi./.lambda.(wavelength);
j.sup.2 is equal to -1.
Since n.sub.e >n.sub.i :
.DELTA. is the thickness of the crystal layer ##EQU3## Comparing
the expression (8) with the standard expression for a thin lens:
##EQU4## the focal length of the lens is identified as: ##EQU5## By
choosing voltages such that the index varies between n.sub.e and an
intermediate value n.sub.i >n.sub.o, the focal length of the
lens can be adjusted between infinity and a minimum value, namely:
##EQU6##
A lens may now be constructed with the aid of equation (10) and
index of refraction data from the response curve shown in FIG. 2.
Given the desired focal length, cell thickness and distance of the
various electrodes from the Z-axis, equation (10) can be used in
conjunction with the response curve to provide a reading of the set
of required voltages. Using well known microprocessor control
techniques, the lens can be quickly and accurately focused and can
also be corrected electrically to overcome the adverse affects of
aberrations to provide for near diffraction limited
performance.
The cell 10 is housed in a package generally referenced 32 (FIG. 6)
which protects the cells and provides a means by which the control
electrodes are connected to a voltage source. To better facilitate
packaging, the plates forming the front and rear windows 11 and 12
are extended beyond the side margins of the cell to create a
cruciform structure as shown in FIG. 6. The control electrodes are
passed out of the chamber to either side thereof and are mounted in
staggered rows on each of the extended aprons 48 and 49. The aprons
48 and 49 are seated, in assembly, upon two raised connector pads
35 and 36. The pads are mounted upon a printed circuit board 37
that forms the bottom section of the package. The pads are
preferably "zebra" connectors having conductive graphite strips
38--38 embedded in a rubber base. In assembly, each strip is seated
in contact with one of the control electrodes and serves to place
the electrode in electrical communication with one of a series of
terminals 39--39 printed along the side edges of the board. The
pads cushion the extended aprons and resiliently support the cell
over a centrally positioned opening 30.
The top section 40 of the package generally complements the bottom
section. The top section also contains a central opening 41 having
a pair of opposed resilient pads disposed along its upper and lower
margins. The pads are adapted to seat in contact against the two
extended aprons 44 and 45 of rear window 12 thus further securing
the cell within the package. The top section of the package
contains a single terminal 46 which is connected to the common
electrode 16. In practice the common electrode may be placed at a
desired operating level or is grounded. Any suitable closure device
may be used to secure the sections in assembly.
As illustrated in FIG. 4, the electrode terminals of package 32 are
connected to a microprocessor 50 through control circuit 51.
Programmed data from the processor is used to place each of the
electrodes at a predetermined voltage to develop the index of
refraction profile needed to bring an incoming light wave to focus.
An input terminal 52 can be operatively connected to the
microprocessor to permit data stored in the processor to be
upgraded or modified.
FIG. 5 illustrates control circuitry used to set each control
electrode at a desired value. Each electrode terminal 39 printed on
circuit board 37 is connected to a shift register 57 through a
digital to analog converter 58 containing a multiplier (not shown).
Digital information is forwarded from the processor via data line
55. An address line 56, which is coupled to the shift register,
identifies the specific information that the register is to
process. Once identified this information is entered into the
register and temporarily stored therein. The stored data is passed
on to the converter in response to a clock pulse signal provided by
clock line 59. A square wave generator 60 is connected by line 61
to the converter and provides the necessary supply voltage to the
electrode. The supply voltage input is combined with the shift
register input in the multiplier circuit which, in turn, produces a
voltage output that is adjusted to the desired level. The output
from the converter is coupled to the electrode terminal through a
capacitor-resistor network 63 which places the electrode voltage at
some desired bias level.
As can be seen, the voltage applied to each electrode can now be
independently maintained to provide point to point control over the
index of refraction of the liquid crystal. Using well known
microprocessing techniques, the index of refraction profile can be
changed in response to corresponding spacial variations in the
control field to change the focal length of the lens. Accordingly,
the lens can be readily focused without the aid of mechanical
linkages or the like. The lens also exhibits all the favorable
characteristics of most liquid crystal devices in that it can be
operated at extremely low voltages while consuming very little
power.
Ideally, the index of refraction profile generated by the lens
should be spherical in order for the system to focus arbitrary
polarized light entering the lens at a plane. This can be
accomplished by the two stage lens system shown in FIG. 7 having a
front cell 65 and a back cell 66 that are mounted in series along
the axis 63 of the system. Each cell is constructed in the manner
described above; however, the control electrodes 67 are configured
on the front windows of the cells in a circular bullseye pattern
centered upon the axis. Although shown exploded, it should be
understood that the cells are packaged, in assembly, in abutting
contact to form a single unit whereby light entering the front cell
will exit the back without passing through another medium such as
air.
The front cell is arranged to influence X-polarized light while the
rear cell is arranged to influence Y-polarized light. This is
achieved by rubbing the interior surfaces of the front cell so that
the liquid crystal molecules, which are shown schematically at 72,
are preferentially aligned in the X-direction. The surfaces of the
rear cell are similarly rubbed to align the molecules along the
Y-axis as illustrated at 73. Staging the cells as shown thus allows
the index of refraction of the liquid crystal to be contoured
electrically to produce a spherical lens response. Furthermore, the
phase front of the lens system is electrically controlled to
provide adaptive lens capabilities and to permit the lens to be
corrected for aberrations and/or other lens defects.
A spherical lens capability can also be achieved by cascading four
liquid crystal cells as shown in FIG. 8 wherein the electrodes
contained in each cell are configured in linear rows rather than
being symmetrical about the axis 84 of the system. In this
embodiment, the front two cells 80 and 81 of the lens system form a
first unit 85 that influences incoming X-polarized light. The two
rear cells 82 and 83 form a second unit 86 that influences
Y-polarized light. The first cell in each unit contains control
electrodes that are disposed along the Y-axis while the electrodes
of the second cell are disposed along the X-axis.
Each cell in the first unit 85 is rubbed to preferentially align
the liquid crystal molecules 87--87 in the X-direction. The cells
making up the second unit 86, on the other hand, are rubbed to
preferentially align the liquid crystal molecules 88--88 in the
Y-direction. As can be seen, the first unit is able to be
electrically controlled to produce the desired phase transfer in
X-polarized light while the second unit similarly influences
Y-polarized light. The net result again is to produce a spherical
lens capability. However, the use of linear electrodes avoids many
of the masking and fabricating difficulties associated with
symmetrical electrodes and also allows the electrodes to be
connected to the input terminals without crossing the connectors
over the electrodes.
Referring once again to FIG. 4, the liquid crystal device 10 of the
present invention is further controlled electrically to correct for
aberrations. This is achieved by adjusting the voltage applied to
selected electrodes. A photoelectric sensor 90 is positioned so
that it can view an image 91 created by the device at the image
plane 93 of the lens 10. A signal from the sensor is delivered to a
sensor controller 94 which compares the sensed image information
with a predetermined optimum value. In the event the sensed image
information is less than optimum, the controller, acting through
the microprocessor, adjusts the voltage on the appropriate
electrode or electrodes to bring the image to a desired operating
level. Through use of the sensing system, the lens can be adapted
to provide near diffraction limited performance.
It should be further noted that through use of the linear electrode
arrangement shown in the lens system illustrated in FIG. 8, the
device can be made to scan or translate electrically in a plane
perpendicular to the axis 84 of the system using well known
microprocessor techniques. It should be understood that in practice
a relatively large number of electrodes per unit area of each cell
are utilized in the present invention. The electrodes, and the
spacing therebetween, may be on the order of between one and twenty
microns wide which is well within the capability of present day
technology. Accordingly, a lens can be created on a small region of
the cell, as depicted at 89 in FIG. 8, and the lens electrically
moved in both the X-direction and the Y-direction to either provide
a scanning capability to the system or to translate the image as
desired.
While this invention has been described with reference to the
structure disclosed herein, it is not confined to the details set
forth and this application is intended to cover any modifications
or changes as may come within the scope of the following
claims.
* * * * *